Fuel cell electrolyte matrix and method for manufacturing the same

ABSTRACT

A method for manufacturing a fuel cell electrolyte matrix comprises the steps of providing a fuel electrode and oxidizing electrode respectively coated with a catalyst layer on one side, forming a layer of powdery electrolyte-resistive material on the surface of at least one of the catalyst layers, coating a paste layer prepared from acid electrolyte and powdery electrolyte-resistive material on the powder layer or catalyst layer of the fuel electrode and/or the powder layer or catalyst layer of the oxidizing electrode, tightly superposing the fuel electrode and oxidizing electrode on each other with the paste layer and powder layer interposed therebetween, thereby forming an electrolyte matrix between both electrodes.

BACKGROUND OF THE INVENTION

This invention relates to a fuel cell and more particularly to theelectrolyte matrix of the fuel cell and a method for manufacturing thesame.

A fuel cell is a device intended to generate direct current by causingan easily oxidizable gas (fuel gas) such as hydrogen and an oxidizinggas such as oxygen to electrochemically react with each other in aproper electrolyte. That type of fuel cell which is put to practicalapplication is constructed by stacking a large number of unit cells withan interconnector set therebetween. Each unit cell comprises a pair ofgas diffusion electrodes and an electrolyte matrix which holds anelectrolyte such as a phosphoric acid and is interposed between saidpaired gas diffusion electrodes. A fuel gas supplied to the outersurface of the gas diffusion electrode and an oxidizing gas brought tothe outer surface of the other gas diffusion electrode are made to reactin the each electrolyte-electrode interface, thereby generating a directcurrent. The inside of the paired gas diffusion electrodes are generallyloaded with a catalyst such as platinum in order to accelerate theabove-mentioned reaction.

The performance property of the fuel cell of the above-mentioned type isoften governed by the quality of the electrolyte matrix used. For thestable operation of the fuel cell, therefore, the electrolyte matrix isrequired to meet the following requirements;

(i) The electrolyte matrix should be stable chemically and thermallyunder an operating condition.

(ii) The electrolyte matrix should contain a sufficiently large amountof electrolyte, and further retain a great capacity to hold theelectrolyte.

(iii) The electrolyte matrix should have a high hydrogen ionconductivity.

(iv) The electrolyte matrix should act as insulator of electrons.

(v) The electrolyte matrix should have a sufficiently high bubblepressure to suppress mutual diffusion between the fuel gas and oxidizinggas.

The conventional fuel cell generally comprises an electrolyte matrixformed of a single layer. However, such single layer type electrolytematrix fails to fully meet the above-listed requirements, thusdecreasing the reliability of the conventional fuel cell and also outputvoltage thereof.

The conventional single layer type electrolyte matrix is constructed bycoating phosphoric acid-resistive fine powder of, for example, siliconcarbide or zirconium oxide on the catalyst layer mounted on the gasdiffusion electrode. To be more concrete, the conventional process ofmanufacturing an electrolyte matrix comprises the steps of:

mixing the proper amounts of silicon carbide, binder of fluorocarbonpolymer such as polytetrafluoroethylene, water and other solvents;

applying the mixture over the surface of the catalyst layer coated onthe gas diffusion substrate by means of, for example, rolling, sprayingor screen printing;

drying the mixture to remove the solvent, thereby producing a matrixbody; and finally

impregnating the matrix body with electrolyte such as phosphoric acid.

However, the above-mentioned electrolyte matrix-manufacturing method hasthe drawbacks that though the application of a smaller amount of binderfacilitates the impregnation of the electrolyte in the matrix body,cracks tend to appear in the matrix body when heating is applied toremove the solvent or the gas diffusion electrode is handled; and theoccurrence of cracks in the matrix body results in a decline in thebubble pressure of the electrolyte, gas utilization rate and theperformance of a fuel cell. Further, difficulties accompanying theconventional electrolyte matrix-manufacturing method are that thoughapplication of a larger amount of the binder can suppress the appearanceof cracks in the matrix body, the hydrophobicity of the binder preventsthe electrolyte from being fully carried into the matrix body, therebydecreasing the conductivity of hydrogen ions.

In view of the difficulties experienced in the conventional electrolytematrix-manufacturing method, the present inventors proposed the methodof manufacturing the electrolyte matrix which comprised the steps ofmixing silicon carbide, binder and phosphoric acid in the form of pasteand spreading the paste over the surface of the gas diffusion electrode.However, the above-mentioned electrolyte matrix-manufacturing methodpreviously proposed by the present inventors which indeed provedprominently useful is still accompanied with the drawbacks that a largercontent of phosphoric acid in the paste intended for improvement of thehydrogen ion conductivity of the electrolyte matrix leads to a rise inthe fluidity of the paste. This increased paste fluidity is accompaniedwith further problems that when a unit cell is constructed bycompressing a pair of gas diffusion electrodes with the paste interposedtherebetween or after this step, the paste leaks crosswise from the unitcell. Such objectionable event results in the difficulties that theelectrolyte matrix is reduced in thickness; the bubble pressure of theelectrolyte decreases; and partial short-circuiting takes place betweenthe paired gas diffusion electrodes. After all, previous method ofmanufacturing an electrolyte matrix from the above-mentioned paste wasstill accompanied with the drawback that the fuel cell eventuallydecreased in performance.

SUMMARY OF THE INVENTION

It is accordingly an object of this invention to provide a fuel cellelectrolyte matrix, which allows for the impregnation of a large amountof electrolyte without reducing the bubble pressure thereof andpreserves the high conductivity of hydrogen ions, thereby assuring thehigh performance of the resultant fuel cell.

Another object of the invention is to provide a simple and practicablemethod of manufacturing said electrolyte matrix. To attain theabove-mentioned objects, this invention provides the method ofmanufacturing an electrolyte matrix for a fuel cell, which comprises thesteps of providing a fuel electrode and oxidizing electrode, each ofwhich is coated with a catalyst layer on one side, forming a layer ofpowdery electrolyte-resistive material on at least one of the catalystlayers, applying a paste prepared from a mixture of acid electrolyte andpowdery electrolyte-resistive material to the powder layer or catalystlayer of the fuel electrode and/or on the powder layer or catalyst layerof the oxidizing electrode, and tightly superposing the fuel electrodeand oxidizing electrode with the paste layer and powder layer interposedtherebetween, thereby producing an electrolyte matrix between theelectrodes.

The electrolyte used in this invention includes phosphoric acid or apolymer, for example, a dimer or tetramer of trifluoromethane sulfonicacid.

The powdery electrolyte-resistive materials used in this inventioninclude silicon carbide, tungsten carbide, silicon nitride, zirconiumoxide, tantalum pentoxide, zirconium phosphate and silicon phosphate anda mixture of two or more of the above-listed materials. Theabove-mentioned powders are chosen to have a particle size of 0.5 to 5microns. The powders need not be limited to the particulate type, butmay be of the fibrous type, or be formed of both particulate and fibroustypes. It is possible to form the powder layer on the catalyst layerspread on the fuel electrode as well as on the catalyst layer coated onthe oxidizing electrode or on either of these catalyst layers. Thepowder layer can be formed by coating the above-mentioned catalyst layerwith a paste composition prepared from 100 parts by weight of powder, 3to 10 parts by weight of binder and 50 to 200 parts by weight ofsolvent, drying the paste composition in the open air at a temperatureof 80° to 150° C. for 1 to 3 hours, subjecting the dried pastecomposition to heat treatment in a non-oxidizing atmosphere at atemperature of 240° to 320° C. for 15 to 60 hours. The binder includes afluorocarbon polymer such as polytetrafluoroethylene andfluoroethylene-propylene copolymer. The solvent includes water,polyethylene glycol and carboxymethyl cellulose.

Like the powder layer, the paste layer can be spread on the catalystlayer or powder layer of the fuel electrode and the catalyst layer orpowder layer of the oxidizing electrode, or on either of there layers.The paste layer may contain a binder as in the aforementioned case. Insuch case the paste is formed of 100 parts by weight of powder, 80 to250 parts by weight of electrolyte and 2 to 30 parts by weight ofbinder.

An electrolyte matrix formed by the aforementioned process between thefuel electrode and oxidizing electrode comprises:

a first layer formed of tightly connected particles ofelectrolyte-resistive material and electrolyte filled in the spacesdefined between said particles; and

a second layer which is set adjacent to said first layer and formed ofloosely connected particles of electrolyte-resistive material andelectrolyte filled in the spaces defined between said particles.

The electrolyte contained in the first layer was originally squeezed outof the paste layer in a large amount. The amount of said electrolyte perunit weight of particles is generally 2 to 3 times larger, than those ofthe electrolyte contained in the second layer.

When a powder layer is formed by the aforementioned process in thesurface of the catalyst layer of the fuel electrode and that of thecatalyst layer of the oxidizing electrode, the resultant electrolytematrix is formed of three layers, that is, the two first layers and oneintervening second layer. When the powder layer is formed on the surfaceof one of said catalyst layers, then the resultant electrolyte matrix isformed of two layers, that is, first and second layers.

The electrolyte matrix of this invention has the advantages that evenwhen the powder layer previously coated on the surface of the catalystlayer of the electrode is cracked while it is dried or handled, thecracks are fully filled with the subsequently coated paste to provide aperfect electrolyte matrix, thereby eliminating the possibility of thebubble pressure being reduced; the powder layer contains an acidelectrolyte and is free from fluidity, and consequently can be formedprecisely with a prescribed thickness; as a result, the fuel electrodeand oxidizing electrode are spaced from each other at a desireddistance; and the electrolyte matrix is effectively prevented from beingexcessively thinned, thereby suppressing a decline in the bubblepressure and consequently the occurrence of local short-circuitingbetween both fuel electrode and oxidizing electrode. Therefore, theelectrolyte matrix of the invention makes a great contribution to theimprovement of the performance of a fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are sectional views showing the successive steps of anelectrolyte matrix-manufacturing method embodying this invention;

FIG. 2 is a fractional sectional view of an electrolyte matrix producedby the steps shown in FIGS. 1A to 1F; and

FIG. 3 is a graph comparing the output performance of a fuel cellprovided with an electrolyte matrix embodying this invention and thatprovided with the conventional electrolyte matrix.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Description may now be given with reference to the accompanying drawingsof an electrolyte matrix-manufacturing method embodying this invention.FIGS. 1A to 1F are sectional views showing the sequential steps of saidelectrolyte matrix-manufacturing method. As shown in FIGS. 1A to 1D,there were first provided a fuel electrode 1 coated with, for example, aplatinum catalyst layer 3 and an oxidizing electrode 2 similarly coatedwith, for example, a platinum catalyst layer 4. A paste compositionprepared by mixing 100 parts by weight of silicon carbide powder havinga smaller size than 10 microns, 5 parts by weight ofpolytetrafluoroethylene as a binder and a proper amount of water wascoated on the surface of a catalyst layer 3 deposited on the fuelelectrode 1 and another catalyst layer 4 spead on the oxidizingelectrode 2 by means of, for example, the doctor blade method. The wholemass was preliminarily dried at 100° C. for 2 hours, and later subjectedto heat treatment at 250° C. for about 30 minutes in a non-oxidizingatmosphere of, for example, nitrogen gas or argon gas. As shown in FIGS.1B and 1E, silicon carbide layers 5, 6 were respectively formed on thecatalyst layers 3, 4 with a thickness of 0.1 mm. The silicon carbidelayers 5, 6 thus formed were accompanied by cracks extending from thesurface to the interior.

A paste composition prepared by mixing 100 parts by weight of siliconcarbide powder having a smaller size than 10 microns, 130 parts byweight of 100% phosphoric acid and 3 parts by weight ofpolytetrafluoroethylene was coated with a thickness of 0.2 mm on, forexample, the silicon carbide layer 5 mounted on the fuel electrode 1(FIG. 1C). Later, a silicon carbide layer 6 partly constituting theoxidizing electrode 2 was superposed in a previously deposited pastelayer 7 (FIG. 1F). Pressure was applied to both sides of the laminatedmass to provide a unit cell consisting of the integrally assembled fuelelectrode 1 and oxidizing electrode 2. Thus, an electrolyte matrix bodyimpregnated with an electrolyte of phosphoric acid was formed betweenthe fuel electrode 1 and oxidizing electrode.

FIG. 2 is an enlarged cross sectional view of the electrolyte matrixthus prepared. An electrolyte matrix 10 formed between the catalystlayer 3 of the fuel electrode 1 and the catalyst layer 4 of theoxidizing electrode 2 consists of a pair of first layers 15, 16 and anintervening layer 17. The first layers 15, 16 correspond to the siliconcarbide layers 5, 6 of FIG. 1F, and the second layer 17 corresponds tothe paste layer 7 of FIG. 1F. FIG. 1F shows the condition of thelaminated mass before it is tightened. Therefore, the silicon carbidelayers 5, 6 are not yet impregnated with a large amount of electrolyte.When the mass of FIG. 1F is vertically squeezed, the electrolytecontained in the paste layer 7 is forcefully brought into the siliconcarbide layers 5, 6 without leaking sidewise. As a result, the firstlayers 15, 16 of FIG. 2 are impregnated with a large amount ofelectrolyte. With the paste layer 7 from which the electrolyte wasextracted, the silicon carbide particles are carried toward the centerof the laminated mass, causing said paste layer 7 to be changed into asecond layer 17 having a thickness half the original level, that is 0.1mm. Now, therefore, said second layer 17 is impregnated with a smalleramount of electrolyte than the first layers 15, 16. The particles of thesilicon carbide layers 5, 6 are tightly connected together by binder andthus prevented from making a relative motion. Even after being squeezed,therefore, the silicon carbide layers 5, 6 show little change inthickness. Therefore, said silicon carbide layers 5, 6 are turned intofirst layers 15, 16 in which a large amount of electrolyte is held inthe cells formed among the particles. An amount of electrolyte held perunit weight of silicon carbide contained in the first layers 15, 16 wasshown to be 1.5 times that which was retained in the second layer 17.

As previously described, a large number of cracks 18 appeared in thefirst layers 15, 16. However, said cracks were filled with the pasteextracted from the paste layer 7 by squeezing. Consequently, theelectrolyte matrix was saved from a decline in bubble pressure whichmight otherwise occur due to the presence of said cracks.

For evaluation of an electrolyte matrix embodying this invention, theundermentioned unit cells were manufactured as controls. Control A wasprepared by the steps of directly impregnating the silicon carbidelayers 5, 6 of FIGS. 1B and 1E with 100% phosphoric acid, superposingelectrodes 1, 2 on each other and tightening them together to provide aunit cell. Control B was manufactured by the steps of tighteningtogether the catalyst layers 3, 4 of the electrodes 1, 2 with the pastelayer 7 interposed therebetween, without forming the silicon carbidelayers 5, 6. Measurement was made of the bubble pressure of anelectrolyte matrix and examinations were made if a squeeze-out of theelectrolyte and semi-short circuiting of the unit cell occur withrespect to the unit cells represented by Controls A, B and a unit cellembodying this invention. The results are given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                Bubble pressure                                                                          Squeeze-out of                                                                            Semi-short                                             (kg/cm.sup.2)                                                                            electrolyte circuiting                                     ______________________________________                                        Example    0.5 to 1.2  Small       None                                       Control A 0.05 to 0.2  Practically None                                                              none                                                   Control B 0.01 to 0.4  Large       Slightly                                                                      noticeable                                 ______________________________________                                    

As seen from Table 1 above, a unit cell provided with an electrolytematrix embodying this invention is far more excellent than the unitcells represented by Controlls A, B.

Three different fuel cells representing the present invention andControls A, B were respectively assembled from a grapn of 10 stackedunit cells (each measuring 20×20 cm²) by interposing therebetween aninterconnector provided with a gas passage groove on both sides. It wasdetermined that changes with time in the output voltage of these threefuel cells at an operating temperature of 200° C. The results are setforth in the curve diagram of FIG. 3. Curve a denotes changes with timein the output voltage of a fuel cell embodying this invention; curve bshows similar data observed in Control A; and curve c indicates similardata obtained from Control B. Data given in FIG. 3 were obtained byconducting determination under the condition in which hydrogen gas wasused as a fuel gas, air was applied as an oxidizing gas, and the currentdensity was set at 200 mA/cm². The determination made with respect tothe fuel cell represented by Control A indicated noticeable variations.Nontheless, the results of said determination may be approximatelyexpressed as curve b.

FIG. 3 proves that the fuel cell embodying this invention generates verystable output voltage over a long period of operation. In contrast, thefuel cell represented by Control A has the drawback that the low bubblepressure of the electrolyte matrix leads to an unstable output voltage.The fuel cell denoted by Control B is also accompanied by difficultiesthat though showing substantially as stable an output voltage on thisinvention during an operation period of about 1000 hours, the fuel cellof Control B shows a gradual decline in the output voltage when operatedlonger than 1000 hours. This undesirable event is supposed to arise fromthe fact that the squeeze-out of the paste results in a decrease in theamount of electrolyte prepared from phosphoric acid; the phosphoric acidis entrained by the effluent gas; and the water content of thephosphoric acid is undesirably evaporated.

As clearly inferred from the result of the above-mentioneddetermination, the electrolyte matrix-manufacturing method of thisinvention is prominently adapted to provide an excellent fuel cell by asimple process with great ease, thereby offering a great industrialadvantage.

What is claimed is:
 1. A method for manufacturing a fuel cellelectrolyte matrix which comprises the steps of:providing a fuelelectrode and oxidizing electrode respectively coated with a catalystlayer on one side; forming a powder layer prepared fromelectrolyte-resistive material on the surface of at least one of saidcatalyst layers; spreading a paste layer prepared from a mixture of acidelectrolyte and powdery electrolyte-resistive material on the powderlayer or catalyst layer coated on the fuel electrode and/or the powderlayer or catalyst layer coated on the oxidizing electrode; tightlysuperposing the fuel electrode and oxidizing electrode with the pastelayer and powder layer interposed therebetween, thereby forming anelectrolyte matrix between both electrodes.
 2. The method according toclaim 1, wherein the electrolyte is formed of phosphoric acid or apolymer of trifluoromethane sulfonic acid; and the electrolyte-resistivematerial is at least one selected from the group consisting of siliconcarbide, tungsten carbide, silicon nitride, zirconium oxide, tantalumpentoxide, zirconium phosphate and silicon phosphate.
 3. The methodaccording to claim 1, wherein the powder layer is formed by the steps ofcoating a composition consisting of electrolyte-resistive material,binder therefor and solvent of said binder on the surface of at leastone of said catalyst layers, and then carrying out the heat-treatment ofthe whole mass.
 4. The method according to claim 3, wherein the binderis the fluorocarbon polymer, and the solvent is water.
 5. The methodaccording to claim 1, wherein the paste contains the binder.
 6. Themethod according to claim 5, wherein the binder is the fluorocarbonpolymer.